Using a 25hp VFD on a 30hp motor

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bbaumer

bbaumer

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I was asked early this morning if it was OK to use a 25 hp rated variable speed drive on a 30 hp motor as "the motor won't ever see full load for the application".

I poked around art. 430 and the sections that apply to adjustable speed drives and did not see that this would be prohibited.

Any opinions on this or did I miss something?

Thanks.
 
I will also add this is already installed this way. The drive is being used on a supply fan. The installer used a drive already on hand to get it up and running quick and to avoid having to purchase a new drive.

It was brought to my attention this morning and I am trying to find a code reference to back up what will be my direction to replace the drive with the "right" one.

"Because I said so" isn't a very good reason. It will be hard to argue overload when the drive has built in overload protection for one and two, the drive will likely cut out on the high duct static pressure safety circuit before it is overloaded anyway.

Thanks.

BTW I am looking for an NEC reference, not Punjabi or Delhi installation standards

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Sahib said:
It is a standard practice throughout the world.
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What standard practice handbook did you find that it?

I have not found anything in the code that would limit it. You might find something in the instruction manual for the drive if you look hard enough, but I just don't think there is anything really wrong with such an install, everything else being OK.

How is this any different than hooking up a VFD to a motor that is too small to drive the load at full speed, which is a common practice.
 
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bbaumer said:
It will be hard to argue overload when the drive has built in overload protection for one and two, the drive will likely cut out on the high duct static pressure safety circuit before it is overloaded anyway.
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It is just to prevent the unwanted tripping of the drive on overload sensing even during normal operation of the motor, the drive size is made at least equal to the motor size. It is just common sense.

Unfortunately, Punjabi or Delhi installation standards and even NEC can not teach common sense.
bbaumer said:
BTW I am looking for an NEC reference, not Punjabi or Delhi installation standards
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bbaumer said:
I was asked early this morning if it was OK to use a 25 hp rated variable speed drive on a 30 hp motor as "the motor won't ever see full load for the application".

I poked around art. 430 and the sections that apply to adjustable speed drives and did not see that this would be prohibited.

Any opinions on this or did I miss something?

Thanks.
Click to expand...

You could tell them to put a 25HP motor on it. Then you have no issue using a 25HP drive. Or buy an AB, I'll bet they have a Powerflex 70 rated 30HP general duty, 25HP heavy duty. That would cover your bases.

I honestly think they are splitting hairs if they are worried about the slight cost difference between a 25HP and 30HP drive. Buying a 30HP and only lightly loading it, may last longer and be cheaper in the long run.
 
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bbaumer said:
I will also add this is already installed this way. The drive is being used on a supply fan. The installer used a drive already on hand to get it up and running quick and to avoid having to purchase a new drive.

It was brought to my attention this morning and I am trying to find a code reference to back up what will be my direction to replace the drive with the "right" one.

"Because I said so" isn't a very good reason. It will be hard to argue overload when the drive has built in overload protection for one and two, the drive will likely cut out on the high duct static pressure safety circuit before it is overloaded anyway.

Thanks.

BTW I am looking for an NEC reference, not Punjabi or Delhi installation standards

View attachment 8128
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Funny but a low blow lol.
 
Cow said:
You could tell them to put a 25HP motor on it. Then you have no issue using a 25HP drive. Or buy an AB, I'll bet they have a Powerflex 70 rated 30HP general duty, 25HP heavy duty. That would cover your bases.

I honestly think they are splitting hairs if they are worried about the slight cost difference between a 25HP and 30HP drive. Buying a 30HP and only lightly loading it, may last longer and be cheaper in the long run.
Click to expand...

No hair splitting to be done. A "free" 25 hp drive you already have sitting on the shelf not being used, is cheaper than buying a new 30hp drive.
 
bbaumer said:
I was asked early this morning if it was OK to use a 25 hp rated variable speed drive on a 30 hp motor as "the motor won't ever see full load for the application".

I poked around art. 430 and the sections that apply to adjustable speed drives and did not see that this would be prohibited.

Any opinions on this or did I miss something?

Thanks.
Click to expand...
I can't comment on code implications but, within some constraints, you can.

One thing to note is that a totally uncloaded cage motor typically takes around 30% of full load current. You can reduce that by running at a reduced V/f ration.
But what does "won't ever see full load" equate? if not 30HP, could it be 29HP? I think, without that sort of information, you have basis for making a informed comment.
 
bbaumer said:
I will also add this is already installed this way.
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So your question of whether it is OK to do so is not whether you can do so but whether it is permissible to do so.
Clearly you can because you have.
And no one seems to be able to find any code/rules that deem it unacceptable.
On that basis, it seems to be OK on both counts.
And, what's the worst that could happen?
 
bbaumer said:
I will also add this is already installed this way. The drive is being used on a supply fan. The installer used a drive already on hand to get it up and running quick and to avoid having to purchase a new drive.
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If it is running OK, why mess with it?

I would bet there are plenty of things that the resources could be better spent on than screwing around with something that is working OK.
 
Don't forget, that in general Article 430 applies even if a VFD is involved.

From the 2005 edition,
430.2 defines a controller as the device that 'is normally used to start and stop a motor by making and breaking motor circuit current'
430.8 requires marking, on the controller, so that the suitablity of the installation can be determined.
430.83 requires controllers to be rated 'not lower than the HP of the motor'.
430.120 says the rest of 430 applies to VFD's, unless noted in 'Part X' which does not address this issue.
 
petersonra said:
If it is running OK, why mess with it?

I would bet there are plenty of things that the resources could be better spent on than screwing around with something that is working OK.
Click to expand...

It took me a lot of years to start picking the battles I wanted to fight. We see so many things that are "wrong" we just can't fix them all, nor do most customers want to pay to correct something that has been successfully wrong for years.
 
jim dungar said:
Don't forget, that in general Article 430 applies even if a VFD is involved.

From the 2005 edition,
430.2 defines a controller as the device that 'is normally used to start and stop a motor by making and breaking motor circuit current'
430.8 requires marking, on the controller, so that the suitablity of the installation can be determined.
430.83 requires controllers to be rated 'not lower than the HP of the motor'.
430.120 says the rest of 430 applies to VFD's, unless noted in 'Part X' which does not address this issue.
Click to expand...

does a VFD actually "make" and "break"? that is normal thought of as something a Cb or switch or contactor would do, i.e.- dry contacts.
 
Bill Wessels

Bill Wessels

What about 2011 NEC 430.83(A)(1)? I think a motor controller (VFD) needs to have a horsepower rating not lower than the horsepowerrating of the motor.

Thanks
Bill Wessels
 
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